This invention relates to the field of tunable diode lasers, and particularly to external cavity tunable diode lasers with high speed wavelength selection capability.
Monochromatic light sources, such as lasers, have broad application in the spectroscopic and telecommunications industry due to their ability to be narrowly tuned to emit a specific wavelength. The applications within these industries are pushing the development of tunable lasers. Current desired characteristics include fast and broadband wavelength tuning, arbitrary wavelength selection, simultaneous selection of multiple wavelengths, no macroscopic mechanical motion, long-term amplitude and spectral stability, low cost, small size and capability for remote programming and control of the output spectrum. Most of these requirements are natural extensions of previous achievements in laser tuning. However, the combined requirements of long term amplitude and optical frequency stability and tunability is a challenging requirement in applications such as telecommunications.
In the telecommunications industry, a set of optical frequencies (wavelengths) has been allocated for optical channels by the ITU committee (an international organization headquartered in Geneva, Switzerland within which governments and the private sector coordinate global telecom networks and services). According to the current standards, the optical channels range from 186 THz (1611.79 nm) to 200.95 THz (1491.88 nm) with fixed channel spacings specified at 100 and 50 GHz covering three bands: C, L, and S. The resulting telecommunications standard, i.e., dense wavelength division multiplexing schema (and similar multiplexing schemas that may be later established as standards and utilized in the future) requires precise and stable laser sources and highly discriminating low loss filters to match to the ITU grid.
One of the popular methods of tuning a laser source is by implementing the Littman-Metcalf external laser cavity configuration. A Littman-Metcalf external cavity configuration is traditionally accomplished with a diffraction grating and a mechanically rotating single mirror used to select the specific wavelength. Using this approach, a high degree of precision in the mirror rotation mechanism is required for wavelength selection, and the tuning process is relatively slow. This original approach, as applied to dye laser technology, is described in detail in the following non-patent prior art: M. G. Littman and H. J. Metcalf, Applied Optics, vol. 17, no. 14, 2224-2227, Jul. 15, 1978, and P. McNicholl and H. J. Metcalf, Applied Optics, vol. 24, no. 17, 2757-2761, Sep. 1, 1985.
An intra-cavity etalon is an additional component that can be used in a laser source for enhanced spectral selectivity of the cavity. An additional Fabry-Perot cavity is provided with partially reflecting mirror surfaces, inserted into the laser resonator, and typically, slightly tilted from the normal to the optical axis of the laser cavity. High precision etalons may be maintained at a constant temperature to ensure dimensional stability. The etalon acts as a wavelength dependent filter restricting the wavelength(s) that can pass through it. As a Fabry-Perot cavity, the etalon imposes a set of longitudinal modes in addition to those of the original cavity. The peaks of transmission correspond to the conditions where constructive interference occurs at both surfaces of the etalon. This happens when the effective thickness of the etalon (i.e., the distance between its partially reflecting surfaces) is an integer multiple of xc2xd the wavelength of the light inside the etalon. The width of the peaks is a function of reflectivity of the etalon""s surfaces; higher reflectivity produces narrower peaks. Fabry-Perot etalons can be designed to produce transmission peaks at specific constant intervals. Outside of the transmission peaks light is mostly reflected from the etalon. If the etalon is slightly tilted in the cavity, this reflection causes loss, thus suppressing any spectral components that are not coincident with the etalon""s transmission peaks.
The use of an etalon for fine tuning of both dye and diode lasers is well known in the art. Use of Fabry-Perot etalons to tune dye lasers is documented in non-patent prior art such as Okada, et al. Applied Optics, vol. 15, 472, 1976; and Okada, et al. Applied Optics, vol. 14, 917, 1975. The effect of an etalon on spectral output of a dye laser is represented in FIG. 1 in which the transmission peaks of the etalon 982 overlay the dye laser spectral gain curve 980. A Fabry-Perot etalon would effectively narrow the laser output to the transmission peak of the etalon within the spectral gain curve of the dye laser.
Use of a Fabry-Perot etalon to increase the stability of a diode laser is documented for example in Applied Optics, vol. 28, 4251, 1989. The effect of an etalon on spectral output of a diode laser is represented in FIG. 2 in which the transmission peaks of the etalon 982 overlay the diode laser spectral gain curve 984. A Fabry-Perot etalon would effectively narrow the laser output to the transmission peaks of the etalon within the spectral gain curve of the diode laser.
The same effect can be implemented continuously over the spectrum and not limited to the etalon transmission peaks by replacing the etalon in the cavity with a Fabry-Perot interferometer (FPI). The FPI acts as an etalon with tunable (or variable) optical length.
However, in either of the situations illustrated by FIGS. 1 and 2, undesired multi-mode emissionxe2x80x94defined by the multiple peaks within the gain curvexe2x80x94may occur, unless some means for additional wavelength selectivity is introduced as part of the laser cavity. Certain particular types of such selectivity, i.e., digital switching means, in combination with the tilted Fabry-Perot etalon, as disclosed in detail herein, do provide relatively coarse wavelength selection to ensure stable single-mode operation of the laser.
As diode lasers have come to replace dye lasers in many applications, a variety of techniques have been applied to tuning diode lasers for implementation in both spectroscopic and telecommunications applications. For example, a variety of U.S. Patents exist for laser tuning with alternative configurations of the mirror at the cavity end. U.S. Pat. No. 4,896,325 discloses an alternative cavity configuration in which a pair of mirrors with narrow discontinuities to provide reflective maxima bound the active cavity. These narrow bands of reflective maxima provide a means for wavelength tuning which is actively controlled by a vernier circuit. U.S. Pat. No. 4,920,541 discloses an external laser cavity configuration of multiple resonator mirrors used to produce multiple wavelength emission from a single laser cavity simultaneously or with a very fast switching time. U.S. Pat. No. 5,319,668 discloses a tunable diode laser with a diffraction grating for wavelength separation and a moveable mirror at the cavity end for wavelength selection. The pivot points are designed to provide an internal cavity length equal to an integer number of half wavelengths at three different wavelengths and an exceptionally close match at all other wavelengths within the tuning range. Alternative tuning arrangements are possible. U.S. Pat. No. 5,771,252 discloses an external cavity continuously tunable wavelength source utilizing a cavity end reflector moveable about a pivot point for simultaneous rotation and translation for wavelength selection.
In addition, several U.S. Patents disclose the use of alternative components in the laser cavity configuration in order to achieve wavelength tuning. U.S. Pat. No. 4,216,439 discloses a spectral line selection technique that utilizes a spectral line selection medium in the gain region of an unstable laser cavity. U.S. Pat. No. 4,897,843 discloses a microprocessor-controlled laser system capable of broadband tuning capabilities by using multiple tuning elements each with progressively finer linewidth control. U.S. Pat. No. 5,276,695 discloses a tunable laser capable of multiple wavelength emission simultaneously or with a very fast switching time between lines by using a laser crystal in the cavity and fine rotation of the cavity end reflective element. U.S. Pat. No. 5,734,666 discloses a wavelength selection apparatus for a laser diode eliminating mechanical motion of a grating by utilizing a laser resonator with piezoelectric-controlled acoustic waves in a crystal for wavelength selection.
U.S. Pat. No. 5,230,005 by Rubino discloses a means of electronically tuning a broadband laser by using a finitely separated spatial light modulator (SLM) and mirror combination in the laser cavity for wavelength selection. The Rubino invention also requires the use of a Fizeau wavemeter as a feedback control element of the system to tune the output wavelength. Rubino""s invention compromises broadband wavelength selection and stability primarily though not exclusively caused by the finite distance between the SLM (wavelength selector) and the mirror (wavelength reflector). This finite distance introduces defocusing of the selected beam and vignetting effects to the system. These problems are somewhat corrected by the use of the interferometric feedback control system, but this is not the ideal solution. The feedback control adds cost and complexity, and increases the wavelength selection time. Ideally, a substantially-better solution lies in using means to provide coplanar wavelength selection and reflection that eliminate the defocusing and vignetting problems and does not require the use of interferometric feedback control.
Coplanarity of the selection and reflection elements is important because, after reflection, the light is sent back precisely along its direction of incidence with extreme precision and remains well collimated. This coplanarity feature is not found in the prior art for laser tuning, and was only first disclosed by commonly-owned and invented U.S. Pat. No. 6,282,213. A well-collimated beam before and after reflection eliminates defocusing and vignetting problems experienced by the prior art. Non-coplanarityxe2x80x94the use a mask separated from a mirror by a non-negligible distance in relation to the wavelengths of the light being selected and reflectedxe2x80x94results in a blurred focal spot on the mirror surface. The size of the blur, on the one hand, is a function of the distance between the mask and the mirror. On the other hand, because each spot on the mirror surface corresponds to a certain wavelength, it can be translated into ambiguity xcex94xcexd of the optical frequency xcexd. By evaluating the optical frequency as a function of the distance between the mask and mirror, the following relationship is established:                     Δυ        =                                            c                              λ                2                                      ·            d                    ⁢                      xe2x80x83                    ⁢          cos          ⁢                      xe2x80x83                    ⁢                      β            ·                          Dh                              f                2                                                                        (        1        )            
where:
xcexd=optical frequency
c=speed of light
xcex=wavelength
d=grating pitch
xcex2=diffraction angle
D=focusing means diameter
h=mask/mirror distance
f=focusing means focal length
A brief example will demonstrate the effect of this distance on a typical system. For example, in the xcex=1500 nm wavelength range, using d=1 xcexcm grating pitch, focal length of the lens xcx9c20 mm, and the lens F/#3, a simple calculation results in the optical frequency ambiguity xcex94xcexd of approximately 100 GHz for h=50 um non-coplanarity. This 100 GHz ambiguity, induced by a separation of only 50 microns, is not acceptable in multiple applications including WDM and DWDM since typical channel spacing is on the same order, i.e. 50 GHz or 100 GHz. Acceptable tolerances would have to be significantly less than the channel spacing in order to differentiate adjacent channels, and in particular, no more than about 10% to 15% of the channel spacing.
Recent non-patent prior art also discloses relevant technology. In SPIE vol. 2482, pp. 269-274 by Zhongqi, Zhang, et al., a microprocessor-controlled tunable diode laser that utilizes a stepper motor to rotate the grating for wavelength tuning is described. In addition, in SPIE vol. 3098, pp. 374-381 by Uenishi, Akimoto, and Nagoka, a tunable laser diode with an external silicon mirror has been fabricated with MEMS technology and has wavelength tunability.
All of the prior art described above is limited in its performance by one or more of the following: requiring mechanical motion, small wavelength range tunability, or limited wavelength-selection order. Especially for applications in spectroscopy and telecommunications, it is very desirable, with limited or no mechanical motion, to achieve broadband wavelength tuning, arbitrary or simultaneous wavelength selection, and long-term amplitude stability. Specifically in telecommunications, the ability to precisely match the ITU grid with long-term amplitude and frequency stability is very much desired, to exceed the capabilities of the above configurations.
Therefore, it would be desirable to provide a tunable diode laser with coplanar wavelength selection and reflection, which in combination with other optical elements in the laser cavity provides a xe2x80x9cperfectxe2x80x9d retroreflector within the tolerances discussed above.
It is also desirable to provide a tunable diode laser with broadband digital wavelength selection capability.
It is also desirable to provide a tunable diode laser with the capability of fast, broadband, digital wavelength selection in arbitrary order.
It is also desirable to provide a tunable diode laser with the capability of fast, broadband, digital wavelength selection in arbitrary order that allows discrete switching between a predetermined series of wavelengths.
It is also desirable to provide a tunable diode laser with simultaneous, digital wavelength selection capability.
It is also desirable to provide a tunable diode laser with the capability of fast switching from the current wavelength to any other wavelength in tuning range.
It is also desirable to provide a tunable diode laser with improved internal focusing which will ultimately improve the total output power.
It is also desirable to provide a tunable diode laser with continuous wave (CW) operation capabilities with fast, broadband, digital line selection capability in arbitrary order that allows discrete switching between or among a predetermined series of wavelengths.
It is also desirable to provide a tunable diode laser with CW operation capabilities with simultaneous, digital line selection capability.
It is also desirable to provide a tunable diode laser with fast, digital line selection capability in arbitrary order that allows discrete switching between wavelengths on the 100 GHz ITU grid, or for other predetermined sets of optical frequencies that may be later established as telecommunications standards and utilized in the future.
It is also desirable to provide a tunable diode laser with fast, digital line selection capability in arbitrary order that allows discrete switching between wavelengths on the 50 GHz ITU grid, as well as other standard grids with smaller channel spacing, or for other predetermined sets of optical frequencies that may be later established as telecommunications standards and utilized in the future.
Disclosed herein is a tunable diode laser configuration that is significantly improved over the prior art. In general, a focusing element, used in combination with a micromirror array (MMA) serves as a xe2x80x9cperfectxe2x80x9d retroreflector in a laser cavity within certain tolerances as discussed herein. This configuration provides both arbitrary and simultaneous line selection capability over a very broad wavelength range. The use of individually controllable micromirrors of the micromirror array improves the overall durability and ruggedness of the device, and the coplanar wavelength selection and reflection inherent in an MMA enables this xe2x80x9cperfectxe2x80x9d retroreflection. It also reduces the size, weight, and cost of the laser, compared to lasers with conventional, Littman-Metcalf cavities and mechanical control of the wavelength. Yet another advantage employing an MMA is the absence of hysteresis and age related deterioration of performance due to wearing out of parts in mechanical systems.
A frequency matching embodiment is realized by combining a laser diode/Fabry-Perot etalon combination with an MMA as herein disclosed, it is further possibly to achieve precise selectivity specifically matched to the ITU grid and similar standard grids. These disclosures can, however, be universally adapted to any laser diode device for any application. The advantages that the present invention provides over the prior art are particularly significant in various spectroscopic and telecommunications applications.
A CW embodiment of the system is realized by fully utilizing most or all of the micromirrors found in a typical 2-dimensional MMA by focusing the elliptical output of the laser diode for a given wavelength on a single column of micromirrors instead of on a single micromirror alone. By using a whole column for a selected wavelength, CW operation can be maintained while individual micromirrors are allowed to periodically cycle to their off state without significant output power reduction to overcome stiction.
All embodiments comprise wavelength separation means for separating incoming light into wavelength-separated light; optical focusing means for focusing the wavelength-separated light into a plurality of single-wavelength focal spots; and locally-controllable reflectivity array means comprising a plurality of individually-controllable localized reflective elements each corresponding to and reflecting one of the plurality of single-wavelength focal spots.
The frequency matching embodiment further comprises wavelength filtering means for substantially allowing wavelengths the incoming light which are separated from one another by a specified optical frequency difference to emerge as filtered light, while substantially barring all other wavelengths. The light separated the wavelength separation means comprises this filtered light.
The CW embodiment further comprises focal spot elongation means for elongating the wavelength-separated light into a plurality of elongated, single-wavelength focal spots; and the locally-controllable reflectivity array means comprises a plurality of sets of a plurality of individually-controllable localized reflective elements; each set corresponding to and reflecting one of the elongated, single-wavelength focal spots.